The present invention relates to an engine speed control system for a vehicle, and more particularly to a system which starts to control the engine idling speed after an interval when the engine operation is changed from driving condition to idling condition.
Heretofore, a closed loop feedback control system has been provided for controlling the idling speed to a desired idling speed by adjusting the amount of air or the amount of the air-fuel mixture to be induced in the engine in dependency on an error signal which is the difference between a desired reference idling speed and the detected idling speed. The feedback control system stops controlling the idling speed in driving condition which is detected by a sensor. After the engine came into the idling operation, the control system starts to control the idling speed after a time lag. The reason why such a delay of control start is necessary is that if the control system starts to control the idling speed immediately after the engine operation has been changed to the idling operation at high speed or after the engine is raced at a high speed then overshoot control in the system will occur, which will cause further control delay to the predetermined idling speed.
In a conventional control system, the period of the control start delay or the time lag is set to a constant period of time. However, the constant time lag is not proper to control the idling speed when the difference between the actual idling speed and the reference idling speed is extremely small or large. That is, when the difference is very small, the time lag is excessively long to control the engine speed to the desired idling speed, and when the difference is very large, the time lag is relatively short, which will cause the abovementioned overshoot control.